The present invention relates to a process for the treatment of a soluble, silicate-bearing material for the recovery of its valuable metal content.
When materials which contain soluble silicate are leached, great difficulties are often encountered in separating the silicic acid from the solution. Silicic acid precipitates easily from a solution as a gel, which is difficult, if not impossible, to remove from the solution by settling or filtering. One example of this is the treatment of a calcine which contains zinc silicate in electrolytic production of zinc. The silicate dissolves in the sulfuric-acid-bearing return acid from the zinc electrolysis. The obtained zinc sulfate solution is purified and fed to the electrolysis, in which the zinc precipitates and the sulfate is regenerated.
Zinc sulfide concentrate constitutes the bulk of the raw materials used in electrolytic production of zinc. The concentrate is roasted and the produced oxide is leached. Zinc is, however, to a considerable extent present as a silicate, or silicate is produced when roasting or calcining silicate-bearing concentrates, in which case SiO.sub.2 reacts with zinc or lead oxide and forms silicates soluble in acid.
In order that such materials could be used for electrolytic production of zinc, it is primarily important to cause the silicic acid to precipitate after the leach in such a form that settling and filtration are technically and economically possible.
Several methods are known for the leach and precipitation. The principle in most methods is to perform the leach using an acid solution, whereby the silicate passes into the solution, whereafter the solution is neutralized using a suitable neutralizing agent (ZnO, CaO). The pH of the solution raises to approx. 5, at which SiO.sub.2 is allowed to precipitate. It has, however, been shown that if the precipitation (neutralization) is performed in batches, it is in any case difficult to control the process so as always to obtain good filtrability.
If the neutralization is performed as a continuous process at a pH of 4.5-6.0, the solution which is obtained from the preceding leach performed at a lower pH (1-3.5) can well be filtered even if the SiO.sub.2 concentration is as high as 50 g/l (German Lay-open Print No. 1,912,545). It is also known that a solution obtained by the leach process described above (pH 1-3.5 and SiO.sub.2 5-60 g/l) is unstable as regards silicic acid and begins to precipitate sooner or later, in this case as a gel which cannot be filtered. This has been shown experimentally. For this reason, operating such a process on a technological scale is very risky, since precautions must always be taken in case of stoppages, in which case the long retention time in the leach reactors and the high SiO.sub.2 concentration cause gel formation. Another disadvantage in such a process lies in that SiO.sub.2 precipitates under such conditions and that, if ZnO is used as the neutralizing agent, a complete leaching yield is never obtained since part of the ZnO remains undissolved together with the SiO.sub.2 precipitate.
The above known process includes a slow leach of silicate material by a "progressive" acid addition method so that the entire leach proceeds at a low acid concentration and the acid concentration never surpasses 1.5-15 g/l. This is performed in batches so that first a neutral slurry is prepared from the silicate material, and then acid is slowly added until the silicate dissolves. In a continuous process the silicate material is fed into the first reactor and acid is then added into each reactor (at minimum 4) so that a successive leach is achieved and the acid concentration does not rise to 1.5-15 g/l until within the last reactor. This requires a very careful control of each reactor. If too much acid is fed into any reactor, the SiO.sub.2 concentration in the solution increases, and if the silicic acid precipitates, a precipitate difficult to filter is obtained. For this reason it is important that the acid concentration remains sufficiently low throughout the leach.